• Journal of the Korean Institute of Gas
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• v.17 no.6
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• pp.27-32
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• 2013

Renewable gas fuels such as biogas and landfill gas are obtained from the biodegradable organic wastes so that they inherently have carbon-neutral nature which can respond global warming. Therefore, attentions are paid to use this renewable gases as a main fuel for internal combustion engines. However, the composition of the fuel varies by its origin or conversion process, it is necessary to make stable combustion and accomplish high efficiency when used in power generating spark ignition (SI) engines. In this study, efforts have been made to investigate the effect of the composition of renewable gas fuel on the engine performance and exhaust emissions. In addition, a new spark plug with a long electrode was tested and compared with a base spark plug as a way to improve engine efficiency and reduce harmful emissions.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.8 no.3
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• pp.442-447
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• 1998

Visible photoluminescence (PL) has been observed generally in the blue and green spectral region from anodically etched porous Ge as well as spark processed Ge. Porous Ge which is prepared by anodic etching without UV light illumination displays the PL peak max of 52 nm (2.38 eV), while porous Ge with UV light illumination exhibits PL peak blue shift to a 470 nm (2.63 eV). Spark processed Ge shows a PL peak max of 520 nm with shoulder peaks at 420 nm and 610nm. The values of energy shift as a function of decreasing temperature between 300 K and 20 K is 0.53 and $1.89\;meVK^{-1}$ for anodic etched Ge without UV illumination and with UV illumination, respectively. On the contrary, no continuous blue shift of PL peak as a function of decreasing temperature is observed for the green luminescing spark processed Ge. From the results of PL as a function of temperature the origin of blue and green luminescing anodically etched Ge as well as spark processed Ge is discussed.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.22 no.5
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• pp.715-721
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• 2018

With the spread of mobile devices, the users of social network services or e-commerce sites have increased dramatically, and the amount of data produced by the users has increased exponentially. E-commerce companies have faced a task regarding how to extract useful information from a vast amount of data produced by the users. To solve this problem, there are various studies applying big data processing technique. In this paper, we propose a collaborative filtering method that applies the tag weight in the Apache Spark platform. In order to elevate the accuracy of recommendation, the proposed method refines the tag data in the preprocessing process and categorizes the items and then applies the information of periods and tag weight to the estimate rating of the items. After generating RDD, we calculate item similarity and prediction values and recommend items to users. The experiment result indicated that the proposed method process large amounts of data quickly and improve the appropriateness of recommendation better.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.18 no.2
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• pp.36-43
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• 2004

In this paper, the frequency spectra from respective mechanical noise signals detected using ultrasonic wave and AE(Acoustic Emission) sensor were analysed to under spark generation between brush and commutator side with arbitrarily 15$^{\circ}$ rotation for brush from the DC motor in operation. Also, the frequency spectra from respective magnetizing noise signals detected using ultrasonic wave and AE sensor were analysed to under neutral point for brush from the DC motor in normal operation. And the analyses and comparison between the mechanical noise signal and magnetizing noise signal of ultrasonic wave with brush location change from the DC motor in operation. As the experimental results, tile mechanical noise signal of ultrasonic wave under spark generation between brush and commutator side with brush location change from the DC motor in operation were increased about 2.5∼3.0 times than magnetizing noise signal of ultrasonic wave form the DC motor in normal operation. Also, the main frequency band for mechanical noise signals of AE under spark generation between brush and commutator side with brush location change from the DC motor in operation, appeared about 1.3[MHz]∼l.5[MHz] by the fast fourier transform.

• Journal of Dental Rehabilitation and Applied Science
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• v.21 no.1
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• pp.33-42
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• 2005

This study was performed to investigate the surface properties and in vitro biocompatibility of electrochemically oxidized Ti-6Al-7Nb alloy by anodic spark discharge technique. Discs of Ti-6Al-7Nb alloy of 20 mm in diameter and 2 mm in thickness were polished sequentially from #300 to 1000 SiC paper, ultrasonically washed with acetone and distilled water for 5 min, and dried in an oven at $50^{\circ}C$ for 24 hours. Anodizing was performed using a regulated DC power supply. The applied voltages were given at 240, 280, 320, and 360 V and current density of $30mA/cm^2$. Hydrothermal treatment was conducted by high pressure steam at $300^{\circ}C$ for 2 hours using a autoclave. Samples were soaked in the Hanks' solution with pH 7.4 at $36.5^{\circ}C$ during 30 days. The results obtained were summarized as follows; 1. The oxide films were porous with pore size of $1{\sim}5{\mu}m$. The size of micropores increased with increasing the spark forming voltage. 2. The main crystal structure of the anodic oxide film was anatase type as analyzed with thin-film X-ray diffractometery. 3. Needle-like hydroxyapatite (HA) crystals were observed on anodic oxide films after hydrothermal treatment at $300^{\circ}C$ for 2 hours. The precipitation of HA crystals was accelerated with increasing the spark forming voltage. 4. The precipitation of the fine asperity-like HA crystals were observed after being immersed in Hanks' solution at $37^{\circ}C$. The precipitation of HA crystals was accelerated with increasing the spark forming voltage and the time of immersion in Hanks' solution. 5. The Ca/P ratio of the precipitated HA layer was equivalent to that of HA crystal as increasing the spark forming voltage and the time of immersion in Hanks' solution.

• Korean Journal of Optics and Photonics
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• v.17 no.1
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• pp.60-67
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• 2006

We constructed a TEA $N_2$ laser which consists of spark gap, pulse type high voltage power supply, Blumlein transmission line circuit, laser tube with Ernst electrode. We observed the self-preionization with an optical fiber in the spark gap and laser tube. The higher voltage power supplied to the Blumlein transmission line circuit, the better preionization was. An U-type transformer yielded better stability and output power than an I-type transformer. The discharge time after triggering a spark gap for the U-type transformer was also short. We obtained the stability of $2.7\%$ and output power of $36{\mu}J$ when the optimum conditions of the laser operation were spark gap distance of 6.0 mm, electrode distance in laser tube of 5.0 mm, $N_2$ gas flow rate in spark gap of 1500 cc/min, $N_2$ gas flow rate in laser tube of 4 ${\iota}$/min, output window reflectivity of $40\%$ and repetition rate of 10 Hz.

• Journal of Powder Materials
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• v.22 no.4
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• pp.283-288
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• 2015

Two sintering methods of a pressureless sintering and a spark-plasma sintering are tested to densify the Fe-TiC composite powders which are fabricated by high-energy ball-milling. A powder mixture of Fe and TiC is prepared in a planetary ball mill at a rotation speed of 500 rpm for 1h. Pressureless sintering is performed at 1100, 1200 and $1300^{\circ}C$ for 1-3 hours in a tube furnace under flowing argon gas atmosphere. Spark-plasma sintering is carried out under the following condition: sintering temperature of $1050^{\circ}C$, soaking time of 10 min, sintering pressure of 50 MPa, heating rate of $50^{\circ}C$, and in a vacuum of 0.1 Pa. The curves of shrinkage and its derivative (shrinkage rate) are obtained from the data stored automatically during sintering process. The densification behaviors are investigated from the observation of fracture surface and cross-section of the sintered compacts. The pressureless-sintered powder compacts show incomplete densification with a relative denstiy of 86.1% after sintering at $1300^{\circ}C$ for 3h. Spark-plasma sintering at $1050^{\circ}C$ for 10 min exhibits nearly complete densification of 98.6% relative density under the sintering pressure of 50 MPa.

• Journal of Dental Rehabilitation and Applied Science
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• v.22 no.1
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• pp.101-110
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• 2006

This study was performed to investigate the surface properties and in vitro biocompatibility of electrochemically oxidized Ti-6Al-7Nb alloy by anodic spark discharge technique. Discs of Ti-6Al-7Nb alloy of 20 mm in diameter and 2 mm in thickness were polished sequentially from #300 to 1000 SiC paper, ultrasonically washed with acetone and distilled water for 5 min, and dried in an oven at $50^{\circ}C$ for 24 hours. Anodizing was performed using a regulated DC power supply. The applied voltages were given at 240, 280, 320, and 360 V and current density of $30mA/cm^2$. Hydrothermal treatment was conducted by high pressure steam at $300^{\circ}C$ for 2 hours using a autoclave. Samples were soaked in the Hanks' solution with pH 7.4 at $36.5^{\circ}C$ during 30 days. The results obtained were summarized as follows; 1. The oxide films were porous with pore size of $1{\sim}5{\mu}m$. The size of micropores increased with increasing the spark forming voltage. 2. The main crystal structure of the anodic oxide film was anatase type as analyzed with thin-film X-ray diffractometery. 3. Needle-like hydroxyapatie (HA) crystals were observed on anodic oxide films after hydrothermal treatment at $300^{\circ}C$ for 2 hours. The precipitation of HA crystals was accelerated with increasing the spark forming voltage. 4. The precipitation of the fine asperity-like HA crystals were observed after being immersed in Hanks' solution at $37^{\circ}C$. The precipitation of HA crystals was accelerated with increasing the spark forming voltage and the time of immersion in Hanks' solution. 5. The Ca/P ration of the precipitated HA layer was equivalent to that of HA crystal as increasing the spark forming voltage and the time of immersion in Hanks' solution.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.21 no.9
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• pp.1731-1737
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• 2017

K Nearest Neighbor Join (KNN Join) is a simple yet effective method in machine learning. It is widely used in small dataset of the past time. As the number of data increases, it is infeasible to run this model on an actual application by a single machine due to memory and time restrictions. Nowadays a popular batch process model called MapReduce which can run on a cluster with a large number of computers is widely used for large-scale data processing. Hadoop is a framework to implement MapReduce, but its performance can be further improved by a new framework named Spark. In the present study, we will provide a KNN Join implement based on Spark. With the advantage of its in-memory calculation capability, it will be faster and more effective than Hadoop. In our experiments, we study the influence of different factors on running time and demonstrate robustness and efficiency of our approach.

• Journal of Powder Materials
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• v.17 no.1
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• pp.13-22
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• 2010

The unique features of spark plasma sintering process are the possibilities of a very fast heating rate and a short holding time to obtain fully dense materials. $\beta$-SiC powder with 0, 2, 6, 10 wt% of $\alpha$-SiC particles (seeds) and 4 wt% of Al-B-C (sintering aids) were spark plasma sintered at $1700-1850^{\circ}C$ for 10 min. The heating rate, applied pressure and sintering atmosphere were kept at $100^{\circ}C/min$, 40 MPa and a flowing Ar gas (500 CC/min). Microstructural development of SiC as function of seed content and temperature during spark plasma sintering was investigated quantitatively and statistically using image analysis. Quantitative image analyses on the sintered SiC ceramics were conducted on the grain size, aspect ratio and grain size distribution of SiC. The microstructure of SiC sintered up to $1700^{\circ}C$ consisted of equiaxed grains. In contrast, the growth of large elongated SiC grains in small matrix grains was shown in sintered bodies at $1750^{\circ}C$ and the plate-like grains interlocking microstructure had been developed by increasing sintering temperature. The introduction of $\alpha$-SiC seeds into $\beta$-SiC accelerated the grain growth of elongated grains during sintering, resulting in the plate-like grains interlocking microstructure. In the $\alpha$-SiC seeds added in $\beta$-SiC, the rate of grain growth decreased with $\alpha$-SiC seed content, however, bulk density and aspect ratio of grains in sintered body increased.